Noctilucent clouds – sometimes called night-shining clouds – are a beautiful seasonal phenomenon. The clouds appear at night and shine silver-blue. They sometimes light up summer night skies at high latitudes – say, about 45 degrees N. or S. – from May through August in the Northern Hemisphere and from November through February in the Southern Hemisphere.

Noctilucent clouds form in the highest reaches of the atmosphere – the mesosphere – as much as 50 miles (80 km) above the Earth’s surface. They’re are thought to be made of ice crystals that form on fine dust particles from meteors. They can only form when temperatures are incredibly low and when there’s water available to form ice crystals.

If you want to see these clouds, remember, you have to be at a relatively high latitude on Earth to see them: between about 45° and 60° North or South latitude. They’re primarily visible when the sun is just below the horizon, say, from about 90 minutes to about two hours after sunset or before sunrise. At such times, when the sun is below the ground horizon but visible from the high altitude of noctilucent clouds, sunlight illuminates these clouds, causing them to glow in the dark night sky.

Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.